Remove read/sync fields from pg_stat_wal and GUC track_wal_io_timing

The four following attributes are removed from pg_stat_wal:
* wal_write
* wal_sync
* wal_write_time
* wal_sync_time

a051e71e28 has added an equivalent of this information in pg_stat_io
with more granularity as this now spreads across the backend types, IO
context and IO objects.  So, keeping the same information in pg_stat_wal
has little benefits.

Another benefit of this commit is the removal of PendingWalStats,
simplifying an upcoming patch to add per-backend WAL statistics, which
already support IO statistics and which have access to the write/sync
stats data of WAL.

The GUC track_wal_io_timing, that was used to enable or disable the
aggregation of the write and sync timings for WAL, is also removed.
pgstat_prepare_io_time() is simplified.

Bump catalog version.
Bump PGSTAT_FILE_FORMAT_ID, due to the update of PgStat_WalStats.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/access/transam/xlogreader.c

@@ -1557,7 +1557,7 @@ WALRead(XLogReaderState *state,
 
 #ifndef FRONTEND
 		/* Measure I/O timing when reading segment */
-		io_start = pgstat_prepare_io_time(track_io_timing);
+		io_start = pgstat_prepare_io_time();
 
 		pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
 #endif